Power MOSFETs (metal oxide semiconductor field effect transistor) are examples of high breakdown voltage semiconductor devices which are used for switching power sources, inverter devices or the like. For example, power MOSFETs are considered to switch high voltages at low-ohmic loads so as to have a very small switching and conduction loss. Power MOSFETs having a small on-resistance (Ron) and a high breakdown voltage when being switched off are desirable. For example, a power MOSFET should withstand a drain to source voltage Vds of some tens to some hundreds volts when being switched off. As a further example, power MOSFETs conduct a very large current which may be up to some hundreds of amperes at a gate-source voltage of about 10 to 20 V at a low voltage drop Vds.
Semiconductor devices having improved transistor characteristics are being searched for. In particular, it is desirable to provide semiconductor devices having increased current efficiency (Ron×A), a steeper sub-threshold slope, a better control of the channel and lower leakage currents. Further, it is desirable to provide a simple process for manufacturing such a semiconductor device.